Also known as "acid precipitation" or "acid deposition", acid rain is precipitation containing harmful amounts of nitric and sulfuric acids formed primarily by sulfur dioxide and nitrogen oxides released into the atmosphere when fossil fuels are burned.

It can be wet precipitation (rain, snow, or fog) or dry precipitation (absorbed gaseous and particulate matter, aerosol particles or dust).

Acid rain has a pH below 5.6. Normal rain has a pH of about 5.6, which is slightly acidic. The term pH is a measure of acidity or alkalinity and ranges from 0 to 14.

1. Rain, or other precipitation that has an excessive concentration of sulfuric or nitric acids.
2. A popular term for the atmospheric deposition of acidified rain, snow, sleet, hail, acidifying gases and particles, as well as, acidified fog and cloud water.
3. Rain, snow, sleet or fog water having a pH less than 5.65.
4. The deposition of acidifying substances from the atmosphere during a precipitation event.